Fluids based on fluorocarbon compounds are widely used in vapor compression heat-transfer systems, in particular air-conditioning, heat-pump, refrigeration or freezing devices. The common feature of these devices is that they are based on a thermodynamic cycle comprising vaporization of the fluid at low pressure (in which the fluid absorbs heat); compression of the vaporized fluid up to a high pressure; condensation of the vaporized fluid to liquid at high pressure (in which the fluid expels heat); and expansion of the fluid to complete the cycle.
The choice of a heat-transfer fluid (which may be a pure compound or a mixture of compounds) is dictated firstly by the thermodynamic properties of the fluid, and secondly by additional constraints. Thus, a particularly important criterion is that of the environmental impact of the fluid under consideration. In particular, chlorinated compounds (chlorofluorocarbons and hydrochlorofluorocarbons) have the disadvantage of damaging the ozone layer. Non-chlorinated compounds are therefore now generally preferred, such as hydrofluorocarbons, fluoro ethers and fluoro olefins.
It is also still necessary to develop other heat-transfer fluids which have a global warming potential (GWP) lower than that of the heat-transfer fluids currently used, and which exhibit equivalent or improved performance levels.
Conventionally, the heat-transfer fluid used for high-temperature heat pumps was CFC-114 (1,2-dichlorotetrafluoroethane). Another chlorinated heat-transfer fluid proposed in this application was HCFC-123 (2,2-dichloro-1,1,1-trifluoroethane).
Given the environmental constraints recalled above, various hydrofluorocarbon-based heat-transfer fluids have been proposed as a replacement for CFC-114 or HCFC-123 for high-temperature heat pumps, for example fluids containing HFC-245fa (1,1,1,3,3-pentafluoropropane) as a mixture with other compounds. In this regard, mention may be made of U.S. Pat. No. 6,814,884, WO 2010/081990, WO 2011/015737 and WO 2011/033200.
There is, however, a need to provide high-temperature heat pumps which operate with fluids which have a lower GWP and which exhibit better energy performance levels.
Moreover, it is known practice to use heat-transfer fluids in the supercritical regime.
Mention may in particular be made of document WO 2008/034828, which describes a motor vehicle air-conditioning system in which CO2 is used in the supercritical regime.
Other documents use heat-transfer fluids in the supercritical regime for generating an electric current (Rankine cycles). This is the case in document WO 2007/131281, in which the compounds used are CO2, n-pentane and some hydrofluorocarbons. It is also the case in document U.S. Pat. No. 6,964,168, in which the compounds used are hydrofluorocarbons or ammonia. It is additionally the case in document WO 96/27739, in which the compounds used are also hydrofluorocarbons or ammonia or else hydrochlorofluorocarbons.
Moreover, hydrofluoroolefins, in particular HFO-1234yf (2,3,3,3-tetrafluoropropene) and HFO-1234ze (1,3,3,3-tetrafluoropropene), are also known to be compounds with a low GWP, which can be used for heat-transfer applications or the like. Documents US 2004/0256594, WO 2005/105947 and WO 2007/002625 mention the use of hydrofluoroolefins in the supercritical regime for quite particular applications such as cleaning, extraction of biological substances, or deposition of catalysts.
There is therefore still a need to provide high-temperature heat pumps which operate with fluids having a lower GWP and which exhibit better energy performance levels, as emphasized above.